1. Field of the Invention
The invention concerns the preparation of surface active agents. In particular, the invention pertains to the use of a catalyzed process for the preparation of directly esterified fatty acyl isethionates (DEFI) formed through the reaction of the alcohol HOR'SO.sub.3 M with the organic acid RCOOH. DEFI has the formula RCOOR'SO.sub.3 M where R is a C.sub.5 -C.sub.18 alkyl group, R' is selected from the group consisting of divalent C.sub.2 -C.sub.4 alkylene, C.sub.6 -C.sub.8 aryl and C.sub.7 -C.sub.8 alkylaryl radicals, and M is an alkali metal cation.
2. The Prior Art
Sodium acyl coconut isethionate esters are major ingredients in many commercial soap bars. Synthesis of these esters by direct esterification has been described in U.S. Pat. Nos. 3,320,292, 3,420,857 and 3,420,858. Typically therein, sodium isethionate is reacted with an excess of coconut fatty acids in the presence of a zinc oxide catalyst. Excess fatty acid is required to maintain high reaction rates, fluidity and suppress foaming. As the reaction proceeds, water, a reaction by-product, distills off alongside a considerable portion of fatty acids. Steam distilled fatty acids are collected and may be recycled to the reaction vessel. Recycling prevents deprivation of fatty acid in the reaction mass. Upon completion of esterification, excess coconut fatty acid is removed by distillation. In the removal of coconut fatty acid, a certain amount of stearic acid is added to the reactor vessel to assist stripping of the more volatile coconut acids. Stearic acid remains behind. It imparts the useful properties of firmness, mildness and lather creaminess to toilet bars.
DEFI in which the fatty acids combined are all coconut derived will produce high lather volume. Where DEFI is formed primarily from high molecular weight fatty acids, e.g., stearic acid, the product lathers poorly. Accordingly, transesterification between coconut isethionate ester and stearic acid is to be avoided. For this reason, stearic acid is added as a distillation chaser only towards the end of the coconut acid esterification with isethioniate and not with the initial charge.
Substitutes for the traditional zinc oxide catalyst which would increase reaction rates are highly desirable. With zinc oxide, rates are hastened only by the increase in catalyst concentration. Unfortunately, increased levels of zinc oxide impart a gritty, sandy feel to the toilet bars.
Recently, Lamberti et al disclosed in U.S. Pat. No. 4,405,526 a type of new "fast" catalyst for the DEFI process. These compounds are formed from a mixture of zinc oxide and organic sulfonic acids. High conversions in relatively short amounts of time are therewith achievable. Unfortunately, the transesterification problem is thereby magnified.
Besides the search for faster catalysts and methods to avoid transesterification, an additional problem concerns controlling excess coconut fatty acids. Surplus acids impair the hardness of soap bars. Moreover, excess coconut acids are to be minimized from a cost perspective.
Another desirable goal in the preparation of DEFI is limitation in the number of processing steps. Time, equipment and thereby costs could well be reduced.
Accordingly, it is an object of this invention to inhibit transesterification between higher fatty acids and coconut isethionate esters.
A further object of this invention is to disclose better catalysts for the DEFI process.
Another object of the invention is to reduce the amount of excess coconut fatty acids needed in the DEFI process.
A still further object of this invention is to reduce the number of steps involved in the DEFI process.